The present invention relates generally to scroll compressors and more specifically to continuous capacity modulation systems of the delayed suction type for such compressors.
Utility summer peak demand limit control has historically been the driving demand behind the need for load shedding for refrigeration compressors. The traditional method used for load shedding has been to have the room thermostat perform an on/off duty cycle of the air conditioning system on the order of every 15 minutes. The disadvantages to this method are that the control and communication hardware cost to implement this system is higher than the savings from demand-side management, and the comfort provided by the system is diminished with long off cycles. Another approach that utilities are using is variable speed air conditioning systems that can modulate capacity and power continuously down to about 75%-80% of capacity. However, not only are variable speed inverters expensive, they also reduce power supply quality through harmonics, thus defeating the utilities original interest. A two-step compressor using a two-speed or a reversing motor is another option, but these systems have limited capability because the motor has to be shut down for 1-2 minutes between speed changes to assure reliability. One possibility to accomplish this load shedding is to utilize a capacity modulated compressor.
A wide variety of systems have been developed in order to accomplish capacity modulation for refrigerant compressors, most of which delay the initial sealing point of the moving fluid pockets defined by the scroll members. In one form, such systems commonly employ a pair of vent passages communicating between suction pressure and the outermost pair of moving fluid pockets. Typically these passages open into the moving fluid pockets at a position within 360xc2x0 of the sealing point of the outer ends of the wraps. Some systems employ a separate valve member for each of these vent passages. The valve members are intended to be operated simultaneously so as to ensure a pressure balance between the two fluid pockets. Other systems employ additional passages to place the two vent passages in fluid communication, thereby enabling use of a single valve to control capacity modulation.
Most recently a capacity modulation system for scroll compressors of the delayed suction type has been developed in which a valving ring is movably supported on the non-orbiting scroll member. An actuating piston is provided which operates to rotate the valving ring relative to the non-orbiting scroll member to thereby selectively open and close one or more vent passages which communicate with selective ones of the moving fluid pockets to thereby vent the pockets to suction. A scroll-type compressor incorporating this type of capacity modulation system is disclosed in U.S. Pat. Nos. 5,678,985 and 6,123,517, the disclosures of which are incorporated by reference. In these capacity modulation systems, the actuating piston is operated by fluid pressure controlled by a solenoid valve. In one version of this design, the solenoid valve and fluid pressure supply and vent lines are positioned externally of the compressor shell. In another version of this design, the solenoid valve is positioned externally of the compressor shell, but the fluid pressure supply and vent lines are positioned internally of the compressor shell.
The object of this invention is to solve the dilemma between demand limit control and the comfort and reliability of the system. The above-discussed capacity modulated systems provide a two-step scroll compressor that can be unloaded to operate at approximately 65% of capacity using a solenoid mechanism. This solenoid mechanism can be activated by the room thermostat directly or it can be activated by a system control module. The low-capacity state, while being referred to as approximately 65%, can actually be designed to be a different percentage if desired. The solenoid is capable of being xe2x80x9cswitched on the flyxe2x80x9d reliably, thus offering continuous capacity control between the low-capacity (i.e., 65%) and full capacity (100%) by pulse width modulation control thereby providing a good balance between peak demand reduction and comfort.
The control solution of the present invention consists of a two-step compressor with its integral unloading solenoid and a Pulse Width Modulated (PWM) control module with software logic which controls the duty-cycle of the solenoid based on an external utility communication signal, a thermostat signal and the outdoor ambient temperature. The duty-cycle can also be controlled based on a load sensor, which can be either a temperature, a pressure, a voltage sensor or a current sensor located within the A/C system which provides an indication of the max-load operating condition of the compressor. The compressor motor remains energized continuously during the duty cycling of the solenoid. Additionally, the evaporator and condenser fan speeds can also be reduced accordingly in proportion to the compressor duty cycle to maximize comfort and system sufficiency.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.